Of information recording media having a recording layer utilizing magnetic, optical, or magneto-optical properties, magnetic disks are available as typical media. Conventionally, as magnetic disk substrates, aluminum substrates have been widely used. However, over recent years, with the demand for the reduction of magnetic head floating amount for recording density enhancement, glass substrates have been increasingly used, which glass substrates exhibit superior surface flatness to aluminum substrates and have less surface defects. Of these, preferably used are glass substrates formed of aluminosilicate glass capable of strengthening substrates by chemical strengthening treatment by ion exchange, because of their enhanced impact resistance and vibration resistance.
In such a glass substrate for an information recording medium, to be capable of high density recording with reduced surface defects, it is necessary that gas bubbles generated in the melting process of glass are allowed to be present in a glass substrate at as lowest level as possible. Conventionally, a method has been commonly employed in which As2O3 and Sb2O3 are contained in a glass component as fining agents, whereby gas bubbles in molten glass are removed (clarified) (for example, refer to Patent Document 1).
However, since As2O3 and Sb2O3 are toxic, from the environmental and health viewpoints, a tendency to regulate the usage thereof is being widespread. Therefore, studies have been made to realize a method to remove gas bubbles in the molten glass without using As2O3 or Sb2O3 serving as fining agents is studied (for example, Patent Document 2), and proposed is a method to remove gas bubbles by depressurizing molten glass.